make THEN step for visibility ignore irrelevant invisible elements

when checking that COUNT elements are in/visible do not check the count of all found element but rather the count of those that are visible or invisible. This also allows to combine providing the COUNT with using the NOT option, e.g. `then 10 elements should not be visible`.

(cherry picked from commit 1157405)

Author: Daniel Gröger

addon-test-support/-private/steps/then.js

@@ -64,20 +64,21 @@ const steps = {
   },
 
   "Then (?:(\\d+) )?$opinionatedElement should (not |NOT )?be visible"(countRaw, [collection/* , label, selector */], no) {
-    assert(`Don't use NOT and number at the same time`, !(no && countRaw));
-
-    let count =
-      no       ? 0                      :
+    const count =
       countRaw ? parseInt(countRaw, 10) :
                  1;
-
-    let m = `Element count`;
-    expect(collection, m).to.have.length(count);
-
-    collection.forEach((element, i) => {
-      m = `Element #${i} (zero-indexed) visibility`;
-      expect(isVisible(element), m).to.be.true;
-    });
+    const countVisible = collection.filter(element => isVisible(element)).length;
+    if (no) {
+      if (countRaw !== undefined) {
+        // check exact match of invisible elements
+        expect(collection.length - countVisible, 'Invisible element count').to.equal(count);
+      } else {
+        // check no element is visible
+        expect(countVisible, 'Visible element count').to.equal(0);
+      }
+    } else {
+      expect(countVisible, 'Visible element count').to.equal(count);
+    }
   },
 
   "Then I should see (NO |no )?(?:(\\d+) )?$opinionatedElement"(no, countRaw, [collection, label, selector]) {